In the field of atomic emission spectroscopy, a gaseous plasma containing the atoms to be examined or measured, is used to excite the atoms sufficiently to cause the atoms to emit radiation at selected wavelengths. In plasma mass spectrometry, the excited atoms are fed into a system for analyzing the atoms electrically. One such device is shown and described in an article by R. D. Satzger et al, entitled "Detection of halogens as positive ions using a He microwave induced plasma as an ion source for mass spectrometry", published in SPECTROCHIMICA ACTA., Vol 42B, No. 5, 1987, pages 705-712. One device commonly used in this field is a microwave induced plasma which generally includes a refractory tube placed along the longitudinal axis of a resonant microwave cavity. A gas, such as helium, containing the atoms is fed into one end of the tube and is excited by the microwave energy supplied to the cavity. For optical spectrometry, the light generated by the atoms is emitted axially from the other end of the tube where it can be examined. For mass spectrometry, the excited atoms are drawn into the mass analyzer where it is examined electronically. A coaxial cable brings power from a microwave supply, generally a magnetron power tube, to the cavity. There is generally a tuner, either in series with the cable, or incorporated into the structure of the cavity. One type of cavity commonly used is shown and described in the Technical Note written by C.I.M. Beenakker, entitled "A cavity for microwave-induced plasmas operated in helium and argon at atmospheric pressure", published in SPECTROCHIMICA ACTA, Vol. 31B, pages 483-486, 1976.
There are several problems with the above described cavity. Both the plasma and the magnetron, can be, under certain circumstances, negative resistance devices. It is common for the tuning to be a critical adjustment, with different settings required for initiating a plasma and for running stably. It is common for tuning adjustments to be necessary after changes in the gas flow. If tuning is slightly in error, several problems can occur. Much of the power can be wasted, by being reflected back to the magnetron. Oscillations in power level can occur, either synchronized with the frequency of the filament power supply (usually 60 Hz) or a super-regenerative oscillation at any frequency from the audio range up to as high as 100 MHz. The system can jump erratically between different magnetron modes, leading to step changes in power. All of these problems lead to errors in measurement.
In cases of more serious mis-tuning, the plasma has a tendency to go out, or not to light at all. Also, large fractions of the power can be dissipated in auxiliary devices, such as tuners and coaxial connectors. In some cases, these devices can be destroyed, either by overheating, or by arcing. Even with apparently optimal tuning, it has been observed that there is substantial heat dissipation in the cables and tuners used with the microwave induced plasma devices. This implies that much of the microwave power is not well coupled into the plasma.
One type of resonant cavity known to those skilled in the art is sometimes referred to as a "coaxial reentrant cavity". The simple use of this cavity shape has not been sufficient to allow independence from the need for tuning. For instance, U.S. Pat. No. 4,575,692 to H. Goldie describes a coaxial reentrant cavity used to support a plasma discharge in a refractory tube. However, since there is still a potential problem with frequency drift, it was necessary to incorporate the cavity into the power-source circuitry, as the main frequency-determining element.
Many attempts have been made, some successful, to ameliorate some of the above problems. However, they have required complex systems for adjusting the tuning and the power or specially built tuners which are more robust and easier to tune. Some workers, such as L. G. Matus, C. B. Boss and A. N. Riddle, REVIEW OF SCIENTIFIC INSTRUMENTS, vol. 54, page 1667 (1983), have removed the need for a separate tuner by the expedient of incorporating a tuning means in the structure of the cavity itself. Clearly, this expedient does not offer the advantage of not having to tune the plasma. Other workers have used thick coupling loops to advantage. But this improvement has not by itself offered freedom from the need for tuning. D. L. Haas, J. W. Carnahan and J. A. Caruso, APPLIED SPECTROSCOPY, vol. 37, page 82 (1982) have used a thick coupling loop, but only to facilitate the use of an internal tuning means. Therefore, it would be desirable to have a microwave induced plasma device which does not require a tuner and is more stable.